System for high efficiency spacecraft orbit transfer

Abstract

The present invention relates generally to systems and methods for transferring a spacecraft from a first orbit to a second orbit. In accordance with one embodiment of the invention, the method comprises calculating thruster-off regions within an orbit transfer in which it is efficient to turn-off spacecraft thrusters, and in those thruster-off regions, turning off the spacecraft thrusters.

1. A method for efficiently transferring a spacecraft to a desired orbit, the method comprising:
 computing a continuous-firing thrust trajectory to achieve an orbit transfer;  
 computing thrust effectiveness values for time intervals over the continuous-firing thrust trajectory;  
 comparing the thrust effectiveness values with a thrust effectiveness threshold value; and  
 computing an intermittent-firing thrust trajectory to achieve the orbit transfer, the intermittent-firing thrust trajectory comprising thruster-on regions where the thrust effectiveness value is about or above the thrust effectiveness threshold value, and thruster-off regions where the thrust effectiveness value is below the thrust effectiveness threshold value.  
 
   
   
     2. The method as recited in  claim 1 , wherein computing the intermittent-firing thrust trajectory comprises:
 determining one or more thruster-off regions for a first orbit revolution;  
 computing a first updated thrust trajectory for the entire orbit transfer using the thruster-off regions identified for the first orbit revolution in the calculation;  
 determining one or more thruster-off regions for a second orbit revolution using the first updated trajectory;  
 computing a second updated thrust trajectory for the entire orbit transfer using the thruster-off regions identified for the first and the second orbit revolutions in the calculation; and  
 continue computing thruster-off regions for each successive orbit revolution and further updated thrust trajectories until a final intermittent-firing thrust trajectory is determined for all orbits of the entire orbit transfer.  
 
   
   
     3. The method as recited in  claim 2 , wherein the thruster-on regions, the thruster-off regions and the final intermittent-firing thrust trajectory are determined prior to carrying out the orbit transfer. 
   
   
     4. The method as recited in  claim 1 , wherein the thrust effectiveness value is calculated according to the equation: 
         Γ   ⁡     (   t   )       =     1   -           λ   6     ⁢     F   .           λ   T     ⁢     z   .         .           
 
   
   
     5. The method as recited in  claim 1 , wherein prior to comparing the thrust effectiveness value with a thrust effectiveness threshold value, the method further comprises determining the thrust effectiveness threshold value. 
   
   
     6. The method as recited in  claim 5 , wherein the thrust effectiveness threshold value is a function of thruster shut-off time, fuel savings and increase in orbit transfer time. 
   
   
     7. The method as recited in  claim 5 , wherein the thrust effectiveness threshold value is denoted Γ 0  and can be solved for by evaluating the integrals 
                     T   1     ⁡     (     Γ   0     )       =       ∫   0   T     ⁢     η   ⁢           ⁢   Γ   ⁢           ⁢     ⅆ   t                         T   2     ⁡     (     Γ   0     )       =       ∫   0   T     ⁢     η   ⁢           ⁢     (     1   -   Γ     )     ⁢           ⁢     ⅆ   t                 ⁢           ⁢   where     ,           η   =       1   ⁢           ⁢   if   ⁢           ⁢   Γ     ≤     Γ   0                   η   =       0   ⁢           ⁢   if   ⁢           ⁢   Γ     >     Γ   0                   
 for values of Γ 0  between 0 and 1 with a reasonable resolution, wherein T 1  gives a relationship between the thrust effectiveness threshold value Γ 0  and a total increase in the orbit transfer time, and wherein T 2  gives a relationship between the thrust effectiveness threshold value Γ 0  and a reduction in firing time.  
 
   
   
     8. The method as recited in  claim 1 , wherein an amount of fuel required to perform the orbit transfer is lower than the amount of fuel required to perform a time-optimal continuous-firing orbit transfer. 
   
   
     9. The method as recited in  claim 1 , wherein an increase in transfer time compared to a time-optimal continuous firing orbit transfer is minimized. 
   
   
     10. The method as recited in  claim 1 , wherein the thrusters are not fired when the orbit change is insensitive to thrusting. 
   
   
     11. The method as recited in  claim 1 , wherein the thrusters are not fired when a required rate of change of thrust trajectory direction is too large for the spacecraft to follow. 
   
   
     12. The method as recited in  claim 1 , wherein the change in orbit comprises a transfer from a launch vehicle injection orbit to a final mission orbit. 
   
   
     13. The method as recited in  claim 1 , wherein the thrusters are not fired when continuously firing the thrusters will not reduce the velocity change required to complete the orbit transfer by at least a threshold amount. 
   
   
     14. A spacecraft orbit transfer system adapted to transfer the spacecraft from a first orbit to a second orbit, the system comprising:
 spacecraft thrusters; and  
 at least one controller adapted to control the spacecraft orbit transfer;  
 the spacecraft orbit transfer system being adapted to: 
 compute a continuous-firing thrust trajectory to achieve an entire orbit transfer;  
 compute thrust effectiveness values for time intervals over the continuous-firing thrust trajectory;  
 compare the thrust effectiveness values with a thrust effectiveness threshold value; and  
 compute an intermittent-firing thrust trajectory to achieve the orbit transfer, the intermittent-firing thrust trajectory comprising thruster-on regions where the thrust effectiveness value is at about or above the thrust effectiveness threshold value and thruster-off regions where the thrust effectiveness value is below the thrust effectiveness threshold value, wherein the spacecraft thrusters are turned-on during the thruster-on regions, and the spacecraft thrusters are turned-off during the thruster-off regions.  
 
 
   
   
     15. The system as recited in  claim 14 , wherein the at least one controller is selected from the group consisting of at least one controller on the spacecraft, at least one controller on the earth, and a combination of at least one controller on the spacecraft and at least one controller on the earth. 
   
   
     16. The system as recited in  claim 14 , wherein the spacecraft orbit transfer system computes the intermittent-firing thrust trajectory by:
 determining one or more thruster-off regions for a first orbit revolution;  
 computing a first updated thrust trajectory for the entire orbit transfer using the thruster-off regions identified for the first orbit revolution in the calculation;  
 determining one or more thruster-off regions for a second orbit revolution using the first updated trajectory;  
 computing a second updated thrust trajectory for the entire orbit transfer using the thruster-off regions identified for the first and the second orbit revolutions in the calculation; and  
 continue computing thruster-off regions for each successive orbit revolution and further updated thrust trajectories until a final intermittent-firing thrust trajectory is determined for all orbits of the entire orbit transfer.  
 
   
   
     17. The system as recited in  claim 16 , wherein the spacecraft orbit transfer system determines the thruster-on regions, the thruster-off regions and the final intermittent-firing thrust trajectory prior to carrying out the orbit transfer. 
   
   
     18. The system as recited in  claim 14 , wherein the thrust effectiveness value is calculated according to the equation: 
         Γ   ⁡     (   t   )       =     1   -           λ   6     ⁢     F   .           λ   T     ⁢     z   .         .           
 
   
   
     19. The system as recited in  claim 14 , wherein the spacecraft orbit transfer system determines the thrust effectiveness threshold value prior to comparing the thrust effectiveness value with a thrust effectiveness threshold value. 
   
   
     20. The system as recited in  claim 19 , wherein the thrust effectiveness threshold value is a function of thruster shut-off time, fuel savings and increase in orbit transfer time. 
   
   
     21. The system as recited in  claim 19 , wherein the thrust effectiveness threshold value is denoted Γ 0  and can be solved for by evaluating the integrals 
                     T   1     ⁡     (     Γ   0     )       =       ∫   0   T     ⁢     η   ⁢           ⁢   Γ   ⁢           ⁢     ⅆ   t                         T   2     ⁡     (     Γ   0     )       =       ∫   0   T     ⁢     η   ⁢           ⁢     (     1   -   Γ     )     ⁢           ⁢     ⅆ   t                 ⁢           ⁢   where     ,           η   =       1   ⁢           ⁢   if   ⁢           ⁢   Γ     ≤     Γ   0                   η   =       0   ⁢           ⁢   if   ⁢           ⁢   Γ     >     Γ   0                   
 for values of Γ 0  between 0 and 1 with a reasonable resolution, wherein T 1  gives a relationship between the thrust effectiveness threshold value Γ 0  and a total increase in the orbit transfer time, and wherein T 2  gives a relationship between the thrust effectiveness threshold value Γ 0  and a reduction in firing time.  
 
   
   
     22. The system as recited in  claim 14 , wherein an amount of fuel required to perform the orbit transfer is lower than the amount of fuel required to perform a time-optimal continuous-firing orbit transfer. 
   
   
     23. The system as recited in  claim 14 , wherein an increase in transfer time compared to a time-optimal continuous firing orbit transfer is minimized. 
   
   
     24. The system as recited in  claim 14 , wherein the thrusters are not fired when the spacecraft orbit change is insensitive to thrusting. 
   
   
     25. The system as recited in  claim 14 , wherein the thrusters are not fired when a required rate of change of thrust trajectory direction is too large for the spacecraft to follow. 
   
   
     26. The system as recited in  claim 14 , wherein the first orbit comprises a launch vehicle injection orbit and the second orbit comprises a final mission orbit. 
   
   
     27. The system as recited in  claim 14 , wherein the thrusters are not fired when continuously firing the thrusters will not reduce the velocity change required to complete the orbit transfer by at least a threshold amount. 
   
   
     28. A spacecraft adapted to transfer from a first orbit to a second orbit, comprising:
 spacecraft thrusters; and  
 an orbit transfer system adapted to transfer the spacecraft from a first orbit to a second orbit, the orbit transfer system comprising at least one controller adapted to control the spacecraft orbit transfer, the spacecraft orbit transfer system being adapted to: 
 compute a continuous-firing thrust trajectory to achieve an entire orbit transfer;  
 compute thrust effectiveness values for time intervals over the continuous-firing thrust trajectory;  
 compare the thrust effectiveness values with a thrust effectiveness threshold value; and  
 compute an intermittent-firing thrust trajectory to achieve the orbit transfer, the intermittent-firing thrust trajectory comprising thruster-on regions where the thrust effectiveness value is at about or above the thrust effectiveness threshold value and thruster-off regions where the thrust effectiveness value is below the thrust effectiveness threshold value, wherein the spacecraft thrusters are turned-on during the thruster-on regions, and the spacecraft thrusters are turned-off during the thruster-off regions.  
 
 
   
   
     29. The spacecraft as recited in  claim 28 , wherein the at least one controller is selected from the group consisting of at least one controller on the spacecraft, at least one controller on the earth, and a combination of at least one controller on the spacecraft and at least one controller on the earth. 
   
   
     30. The spacecraft as recited in  claim 28 , wherein the orbit transfer system computes the intermittent-firing thrust trajectory by:
 determining one or more thruster-off regions for a first orbit revolution;  
 computing a first updated thrust trajectory for the entire orbit transfer using the thruster-off regions identified for the first orbit revolution in the calculation;  
 determining one or more thruster-off regions for a second orbit revolution using the first updated trajectory;  
 computing a second updated thrust trajectory for the entire orbit transfer using the thruster-off regions identified for the first and the second orbit revolutions in the calculation; and  
 continue computing thruster-off regions for each successive orbit revolution and further updated thrust trajectories until a final intermittent-firing thrust trajectory is determined for all orbits of the entire orbit transfer.  
 
   
   
     31. A method for transferring a spacecraft from a first orbit to a second orbit, comprising:
 calculating thruster-off regions within the orbit transfer in which it is efficient to turnoff spacecraft thrusters, based on a comparison for each region of a computed thrust effectiveness value to a thrust effectiveness threshold value; and  
 turning off the spacecraft thrusters in the thruster-off regions during the orbit transfer.